Frontend testing standards and style guidelines

There are two types of test suites encountered while developing frontend code at GitLab. We use Karma with Jasmine and Jest for JavaScript unit and integration testing, and RSpec feature tests with Capybara for e2e (end-to-end) integration testing.

Unit and feature tests need to be written for all new features. Most of the time, you should use RSpec for your feature tests.

Regression tests should be written for bug fixes to prevent them from recurring in the future.

See the Testing Standards and Style Guidelines page for more information on general testing practices at GitLab.

Vue.js testing

If you are looking for a guide on Vue component testing, you can jump right away to this section.

Jest

We use Jest to write frontend unit and integration tests. Jest tests can be found in /spec/frontend and /ee/spec/frontend in EE.

Karma test suite

While GitLab has switched over to Jest, Karma tests still exist in our application because some of our specs require a browser and can't be easily migrated to Jest. Those specs intend to eventually drop Karma in favor of either Jest or RSpec. You can track this migration in the related epic.

Karma is a test runner which uses Jasmine as its test framework. Jest also uses Jasmine as foundation, that's why it's looking quite similar.

Karma tests live in spec/javascripts/ and /ee/spec/javascripts in EE.

app/assets/javascripts/behaviors/autosize.js might have a corresponding spec/javascripts/behaviors/autosize_spec.js file.

Keep in mind that in a CI environment, these tests are run in a headless browser and you don't have access to certain APIs, such as Notification, which have to be stubbed.

Differences to Karma

Jest runs in a Node.js environment, not in a browser. Support for running Jest tests in a browser is planned.
Because Jest runs in a Node.js environment, it uses jsdom by default. See also its limitations below.
Jest does not have access to Webpack loaders or aliases. The aliases used by Jest are defined in its own configuration.
All calls to setTimeout and setInterval are mocked away. See also Jest Timer Mocks.
rewire is not required because Jest supports mocking modules. See also Manual Mocks.
No context object is passed to tests in Jest. This means sharing this.something between beforeEach() and it() for example does not work. Instead you should declare shared variables in the context that they are needed (via const / let).
The following cause tests to fail in Jest:
- Unmocked requests.
- Unhandled Promise rejections.
- Calls to console.warn, including warnings from libraries like Vue.

Limitations of jsdom

As mentioned above, Jest uses jsdom instead of a browser for running tests. This comes with a number of limitations, namely:

See also the issue for support running Jest tests in browsers.

Debugging Jest tests

Running yarn jest-debug runs Jest in debug mode, allowing you to debug/inspect as described in the Jest docs.

Timeout error

The default timeout for Jest is set in /spec/frontend/test_setup.js.

If your test exceeds that time, it fails.

If you cannot improve the performance of the tests, you can increase the timeout for a specific test using setTestTimeout.

import { setTestTimeout } from 'helpers/timeout';

describe('Component', () => {
  it('does something amazing', () => {
    setTestTimeout(500);
    // ...
  });
});

Remember that the performance of each test depends on the environment.

Test-specific stylesheets

To help facilitate RSpec integration tests we have two test-specific stylesheets. These can be used to do things like disable animations to improve test speed, or to make elements visible when they need to be targeted by Capybara click events:

app/assets/stylesheets/disable_animations.scss
app/assets/stylesheets/test_environment.scss

Because the test environment should match the production environment as much as possible, use these minimally and only add to them when necessary.

What and how to test

Before jumping into more gritty details about Jest-specific workflows like mocks and spies, we should briefly cover what to test with Jest.

Don't test the library

Libraries are an integral part of any JavaScript developer's life. The general advice would be to not test library internals, but expect that the library knows what it's supposed to do and has test coverage on its own. A general example could be something like this

import { convertToFahrenheit } from 'temperatureLibrary'

function getFahrenheit(celsius) {
  return convertToFahrenheit(celsius)
}

It does not make sense to test our getFahrenheit function because underneath it does nothing else but invoking the library function, and we can expect that one is working as intended. (Simplified, I know)

Let's take a short look into Vue land. Vue is a critical part of the GitLab JavaScript codebase. When writing specs for Vue components, a common gotcha is to actually end up testing Vue provided functionality, because it appears to be the easiest thing to test. Here's an example taken from our codebase.

// Component script
{
  computed: {
    hasMetricTypes() {
      return this.metricTypes.length;
    },
}

<!-- Component template -->
<template>
  <gl-dropdown v-if="hasMetricTypes">
    <!-- Dropdown content -->
  </gl-dropdown>
</template>

Testing the hasMetricTypes computed prop would seem like a given here. But to test if the computed property is returning the length of metricTypes, is testing the Vue library itself. There is no value in this, besides it adding to the test suite. It's better to test a component in the way the user interacts with it: checking the rendered template.

// Bad
describe('computed', () => {
  describe('hasMetricTypes', () => {
    it('returns true if metricTypes exist', () => {
      factory({ metricTypes });
      expect(wrapper.vm.hasMetricTypes).toBe(2);
    });

    it('returns true if no metricTypes exist', () => {
      factory();
      expect(wrapper.vm.hasMetricTypes).toBe(0);
    });
  });
});

// Good
it('displays a dropdown if metricTypes exist', () => {
  factory({ metricTypes });
  expect(wrapper.findComponent(GlDropdown).exists()).toBe(true);
});

it('does not display a dropdown if no metricTypes exist', () => {
  factory();
  expect(wrapper.findComponent(GlDropdown).exists()).toBe(false);
});

Keep an eye out for these kinds of tests, as they just make updating logic more fragile and tedious than it needs to be. This is also true for other libraries. A rule of thumb here is: if you are checking a wrapper.vm property, you should probably stop and rethink the test to check the rendered template instead.

Some more examples can be found in the Frontend unit tests section

Don't test your mock

Another common gotcha is that the specs end up verifying the mock is working. If you are using mocks, the mock should support the test, but not be the target of the test.

const spy = jest.spyOn(idGenerator, 'create')
spy.mockImplementation = () = '1234'

// Bad
expect(idGenerator.create()).toBe('1234')

// Good: actually focusing on the logic of your component and just leverage the controllable mocks output
expect(wrapper.find('div').html()).toBe('<div id="1234">...</div>')

Follow the user

The line between unit and integration tests can be quite blurry in a component heavy world. The most important guideline to give is the following:

Write clean unit tests if there is actual value in testing a complex piece of logic in isolation to prevent it from breaking in the future
Otherwise, try to write your specs as close to the user's flow as possible

For example, it's better to use the generated markup to trigger a button click and validate the markup changed accordingly than to call a method manually and verify data structures or computed properties. There's always the chance of accidentally breaking the user flow, while the tests pass and provide a false sense of security.

Common practices

These some general common practices included as part of our test suite. Should you stumble over something not following this guide, ideally fix it right away. 🎉

How to query DOM elements

When it comes to querying DOM elements in your tests, it is best to uniquely and semantically target the element.

Preferentially, this is done by targeting what the user actually sees using DOM Testing Library. When selecting by text it is best to use the byRole query as it helps enforce accessibility best practices. findByRole and the other DOM Testing Library queries are available when using shallowMountExtended or mountExtended.

When writing Vue component unit tests, it can be wise to query children by component, so that the unit test can focus on comprehensive value coverage rather than dealing with the complexity of a child component's behavior.

Sometimes, neither of the above are feasible. In these cases, adding test attributes to simplify the selectors might be the best option. A list of possible selectors include:

A semantic attribute like name (also verifies that name was setup properly)
A data-testid attribute (recommended by maintainers of @vue/test-utils) optionally combined with shallowMountExtended or mountExtended
a Vue ref (if using @vue/test-utils)

import { shallowMountExtended } from 'helpers/vue_test_utils_helper'

const wrapper = shallowMountExtended(ExampleComponent);

// In this example, `wrapper` is a `@vue/test-utils` wrapper returned from `mount` or `shallowMount`.
it('exists', () => {
  // Best (especially for integration tests)
  wrapper.findByRole('link', { name: /Click Me/i })
  wrapper.findByRole('link', { name: 'Click Me' })
  wrapper.findByText('Click Me')
  wrapper.findByText(/Click Me/i)

  // Good (especially for unit tests)
  wrapper.findComponent(FooComponent);
  wrapper.find('input[name=foo]');
  wrapper.find('[data-testid="my-foo-id"]');
  wrapper.findByTestId('my-foo-id'); // with shallowMountExtended or mountExtended – check below
  wrapper.find({ ref: 'foo'});

  // Bad
  wrapper.find('.js-foo');
  wrapper.find('.btn-primary');
  wrapper.find('.qa-foo-component');
  wrapper.find('[data-qa-selector="foo"]');
});

It is recommended to use kebab-case for data-testid attribute.

It is not recommended that you add .js-* classes just for testing purposes. Only do this if there are no other feasible options available.

Do not use a .qa-* class or data-qa-selector attribute for any tests other than QA end-to-end testing.

Querying for child components

When testing Vue components with @vue/test-utils another possible approach is querying for child components instead of querying for DOM nodes. This assumes that implementation details of behavior under test should be covered by that component's individual unit test. There is no strong preference in writing DOM or component queries as long as your tests reliably cover expected behavior for the component under test.

Example:

it('exists', () => {
  wrapper.findComponent(FooComponent);
});

Naming unit tests

When writing describe test blocks to test specific functions/methods, use the method name as the describe block name.

Bad:

describe('#methodName', () => {
  it('passes', () => {
    expect(true).toEqual(true);
  });
});

describe('.methodName', () => {
  it('passes', () => {
    expect(true).toEqual(true);
  });
});

Good:

describe('methodName', () => {
  it('passes', () => {
    expect(true).toEqual(true);
  });
});

Testing promises

When testing Promises you should always make sure that the test is asynchronous and rejections are handled. It's now possible to use the async/await syntax in the test suite:

it('tests a promise', async () => {
  const users = await fetchUsers()
  expect(users.length).toBe(42)
});

it('tests a promise rejection', async () => {
  await expect(user.getUserName(1)).rejects.toThrow('User with 1 not found.');
});

You can also simply return a promise from the test function.

Using the done and done.fail callbacks is discouraged when working with promises. They should only be used when testing callback-based code.

Bad:

// missing return
it('tests a promise', () => {
  promise.then(data => {
    expect(data).toBe(asExpected);
  });
});

// uses done/done.fail
it('tests a promise', done => {
  promise
    .then(data => {
      expect(data).toBe(asExpected);
    })
    .then(done)
    .catch(done.fail);
});

Good:

// verifying a resolved promise
it('tests a promise', () => {
  return promise
    .then(data => {
      expect(data).toBe(asExpected);
    });
});

// verifying a resolved promise using Jest's `resolves` matcher
it('tests a promise', () => {
  return expect(promise).resolves.toBe(asExpected);
});

// verifying a rejected promise using Jest's `rejects` matcher
it('tests a promise rejection', () => {
  return expect(promise).rejects.toThrow(expectedError);
});

Manipulating Time

Sometimes we have to test time-sensitive code. For example, recurring events that run every X amount of seconds or similar. Here are some strategies to deal with that:

`setTimeout()` / `setInterval()` in application

If the application itself is waiting for some time, mock await the waiting. In Jest this is already done by default (see also Jest Timer Mocks). In Karma you can use the Jasmine mock clock.

const doSomethingLater = () => {
  setTimeout(() => {
    // do something
  }, 4000);
};

in Jest:

it('does something', () => {
  doSomethingLater();
  jest.runAllTimers();

  expect(something).toBe('done');
});

in Karma:

it('does something', () => {
  jasmine.clock().install();

  doSomethingLater();
  jasmine.clock().tick(4000);

  expect(something).toBe('done');
  jasmine.clock().uninstall();
});

Waiting in tests

Sometimes a test needs to wait for something to happen in the application before it continues. Avoid using setTimeout because it makes the reason for waiting unclear and if used within Karma with a time larger than zero it slows down our test suite. Instead use one of the following approaches.

Promises and Ajax calls

const askTheServer = () => {
  return axios
    .get('/endpoint')
    .then(response => {
      // do something
    })
    .catch(error => {
      // do something else
    });
};

in Jest:

it('waits for an Ajax call', async () => {
  await askTheServer()
  expect(something).toBe('done');
});

in Karma:

it('waits for an Ajax call', done => {
  askTheServer()
    .then(() => {
      expect(something).toBe('done');
    })
    .then(done)
    .catch(done.fail);
});

If you are not able to register handlers to the Promise, for example because it is executed in a synchronous Vue life cycle hook, take a look at the waitFor helpers or you can flush all pending Promises:

in Jest:

it('waits for an Ajax call', () => {
  synchronousFunction();
  jest.runAllTicks();

  expect(something).toBe('done');
});

Vue rendering

To wait until a Vue component is re-rendered, use either of the equivalent Vue.nextTick() or vm.$nextTick().

in Jest:

it('renders something', () => {
  wrapper.setProps({ value: 'new value' });

  return wrapper.vm.$nextTick().then(() => {
    expect(wrapper.text()).toBe('new value');
  });
});

in Karma:

it('renders something', done => {
  wrapper.setProps({ value: 'new value' });

  wrapper.vm
    .$nextTick()
    .then(() => {
      expect(wrapper.text()).toBe('new value');
    })
    .then(done)
    .catch(done.fail);
});

Events

If the application triggers an event that you need to wait for in your test, register an event handler which contains the assertions:

it('waits for an event', done => {
  eventHub.$once('someEvent', eventHandler);

  someFunction();

  function eventHandler() {
    expect(something).toBe('done');
    done();
  }
});

In Jest you can also use a Promise for this:

it('waits for an event', () => {
  const eventTriggered = new Promise(resolve => eventHub.$once('someEvent', resolve));

  someFunction();

  return eventTriggered.then(() => {
    expect(something).toBe('done');
  });
});

Ensuring that tests are isolated

Tests are normally architected in a pattern which requires a recurring setup and breakdown of the component under test. This is done by making use of the beforeEach and afterEach hooks.

Example

  let wrapper;

  beforeEach(() => {
    wrapper = mount(Component);
  });

  afterEach(() => {
    wrapper.destroy();
  });

When looking at this initially you'd suspect that the component is setup before each test and then broken down afterwards, providing isolation between tests.

This is however not entirely true as the destroy method does not remove everything which has been mutated on the wrapper object. For functional components, destroy only removes the rendered DOM elements from the document.

Jest best practices

Introduced in GitLab 13.2.

Prefer `toBe` over `toEqual` when comparing primitive values

Jest has toBe and toEqual matchers. As toBe uses Object.is to compare values, it's faster (by default) than using toEqual. While the latter eventually falls back to leverage Object.is, for primitive values, it should only be used when complex objects need a comparison.

Examples:

const foo = 1;

// Bad
expect(foo).toEqual(1);

// Good
expect(foo).toBe(1);

Prefer more befitting matchers

Jest provides useful matchers like toHaveLength or toBeUndefined to make your tests more readable and to produce more understandable error messages. Check their docs for the full list of matchers.

Examples:

const arr = [1, 2];

// prints:
// Expected length: 1
// Received length: 2
expect(arr).toHaveLength(1);

// prints:
// Expected: 1
// Received: 2
expect(arr.length).toBe(1);

// prints:
// expect(received).toBe(expected) // Object.is equality
// Expected: undefined
// Received: "bar"
const foo = 'bar';
expect(foo).toBe(undefined);

// prints:
// expect(received).toBeUndefined()
// Received: "bar"
const foo = 'bar';
expect(foo).toBeUndefined();

Avoid using `toBeTruthy` or `toBeFalsy`

Jest also provides following matchers: toBeTruthy and toBeFalsy. We should not use them because they make tests weaker and produce false-positive results.

For example, expect(someBoolean).toBeFalsy() passes when someBoolean === null, and when someBoolean === false.

Tricky `toBeDefined` matcher

Jest has the tricky toBeDefined matcher that can produce false positive test. Because it validates the given value for undefined only.

// Bad: if finder returns null, the test will pass
expect(wrapper.find('foo')).toBeDefined();

// Good
expect(wrapper.find('foo').exists()).toBe(true);

Avoid using `setImmediate`

Try to avoid using setImmediate. setImmediate is an ad-hoc solution to run your callback after the I/O completes. And it's not part of the Web API, hence, we target NodeJS environments in our unit tests.

Instead of setImmediate, use jest.runAllTimers or jest.runOnlyPendingTimers to run pending timers. The latter is useful when you have setInterval in the code. Remember: our Jest configuration uses fake timers.

Avoid non-deterministic specs

Non-determinism is the breeding ground for flaky and brittle specs. Such specs end up breaking the CI pipeline, interrupting the work flow of other contributors.

Make sure your test subject's collaborators (e.g., Axios, apollo, Lodash helpers) and test environment (e.g., Date) behave consistently across systems and over time.
Make sure tests are focused and not doing "extra work" (e.g., needlessly creating the test subject more than once in an individual test)

Faking `Date` for determinism

Date is faked by default in our Jest environment. This means every call to Date() or Date.now() returns a fixed deterministic value.

If you really need to change the default fake date, you can call useFakeDate within any describe block, and the date will be replaced for that specs within that describe context only:

import { useFakeDate } from 'helpers/fake_date';

describe('cool/component', () => {
  // Default fake `Date`
  const TODAY = new Date();

  // NOTE: `useFakeDate` cannot be called during test execution (i.e. inside `it`, `beforeEach`, `beforeAll`, etc.).
  describe("on Ada Lovelace's Birthday", () => {
    useFakeDate(1815, 11, 10)

    it('Date is no longer default', () => {
      expect(new Date()).not.toEqual(TODAY);
    });
  });

  it('Date is still default in this scope', () => {
    expect(new Date()).toEqual(TODAY)
  });
})

Similarly, if you really need to use the real Date class, then you can import and call useRealDate within any describe block:

import { useRealDate } from 'helpers/fake_date';

// NOTE: `useRealDate` cannot be called during test execution (i.e. inside `it`, `beforeEach`, `beforeAll`, etc.).
describe('with real date', () => {
  useRealDate();
});

Faking `Math.random` for determinism

Consider replacing Math.random with a fake when the test subject depends on it.

beforeEach(() => {
  // https://xkcd.com/221/
  jest.spyOn(Math, 'random').mockReturnValue(0.4);
});

Factories

TBU

Mocking Strategies with Jest

Stubbing and Mocking

Jasmine provides stubbing and mocking capabilities. There are some subtle differences in how to use it within Karma and Jest.

Stubs or spies are often used synonymously. In Jest it's quite easy thanks to the .spyOn method. Official docs The more challenging part are mocks, which can be used for functions or even dependencies.

Manual module mocks

Manual mocks are used to mock modules across the entire Jest environment. This is a very powerful testing tool that helps simplify unit testing by mocking out modules which cannot be easily consumed in our test environment.

WARNING: Do not use manual mocks if a mock should not be consistently applied in every spec (i.e. it's only needed by a few specs). Instead, consider using jest.mock(..) (or a similar mocking function) in the relevant spec file.

Where should I put manual mocks?

Jest supports manual module mocks by placing a mock in a __mocks__/ directory next to the source module (e.g. app/assets/javascripts/ide/__mocks__). Don't do this. We want to keep all of our test-related code in one place (the spec/ folder).

If a manual mock is needed for a node_modules package, use the spec/frontend/__mocks__ folder. Here's an example of a Jest mock for the package monaco-editor.

If a manual mock is needed for a CE module, place it in spec/frontend/mocks/ce.

Files in spec/frontend/mocks/ce mocks the corresponding CE module from app/assets/javascripts, mirroring the source module's path.
- Example: spec/frontend/mocks/ce/lib/utils/axios_utils mocks the module ~/lib/utils/axios_utils.
We don't support mocking EE modules yet.
If a mock is found for which a source module doesn't exist, the test suite fails. 'Virtual' mocks, or mocks that don't have a 1-to-1 association with a source module, are not supported yet.

Manual mock examples

mocks/axios_utils - This mock is helpful because we don't want any unmocked requests to pass any tests. Also, we are able to inject some test helpers such as axios.waitForAll.
__mocks__/mousetrap/index.js - This mock is helpful because the module itself uses AMD format which webpack understands, but is incompatible with the jest environment. This mock doesn't remove any behavior, only provides a nice es6 compatible wrapper.
__mocks__/monaco-editor/index.js - This mock is helpful because the Monaco package is completely incompatible in a Jest environment. In fact, webpack requires a special loader to make it work. This mock makes this package consumable by Jest.

Keep mocks light

Global mocks introduce magic and technically can reduce test coverage. When mocking is deemed profitable:

Keep the mock short and focused.
Leave a top-level comment in the mock on why it is necessary.

Additional mocking techniques

Consult the official Jest docs for a full overview of the available mocking features.

Running Frontend Tests

For running the frontend tests, you need the following commands:

rake frontend:fixtures (re-)generates fixtures. Make sure that fixtures are up-to-date before running tests that require them.
yarn jest runs Jest tests.
yarn karma runs Karma tests.

Live testing and focused testing -- Jest

While you work on a test suite, you may want to run these specs in watch mode, so they rerun automatically on every save.

# Watch and rerun all specs matching the name icon
yarn jest --watch icon

# Watch and rerun one specific file
yarn jest --watch path/to/spec/file.spec.js

You can also run some focused tests without the --watch flag

# Run specific jest file
yarn jest ./path/to/local_spec.js
# Run specific jest folder
yarn jest ./path/to/folder/
# Run all jest files which path contain term
yarn jest term

Live testing and focused testing -- Karma

Karma allows something similar, but it's way more costly.

Running Karma with yarn run karma-start compiles the JavaScript assets and runs a server at http://localhost:9876/ where it automatically runs the tests on any browser which connects to it. You can enter that URL on multiple browsers at once to have it run the tests on each in parallel.

While Karma is running, any changes you make instantly trigger a recompile and retest of the entire test suite, so you can see instantly if you've broken a test with your changes. You can use Jasmine focused or excluded tests (with fdescribe or xdescribe) to get Karma to run only the tests you want while you're working on a specific feature, but make sure to remove these directives when you commit your code.

It is also possible to only run Karma on specific folders or files by filtering the run tests via the argument --filter-spec or short -f:

# Run all files
yarn karma-start
# Run specific spec files
yarn karma-start --filter-spec profile/account/components/update_username_spec.js
# Run specific spec folder
yarn karma-start --filter-spec profile/account/components/
# Run all specs which path contain vue_shared or vie
yarn karma-start -f vue_shared -f vue_mr_widget

You can also use glob syntax to match files. Remember to put quotes around the glob otherwise your shell may split it into multiple arguments:

# Run all specs named `file_spec` within the IDE subdirectory
yarn karma -f 'spec/javascripts/ide/**/file_spec.js'

Frontend test fixtures

Frontend fixtures are files containing responses from backend controllers. These responses can be either HTML generated from HAML templates or JSON payloads. Frontend tests that rely on these responses are often using fixtures to validate correct integration with the backend code.

Generate fixtures

You can find code to generate test fixtures in:

spec/frontend/fixtures/, for running tests in CE.
ee/spec/frontend/fixtures/, for running tests in EE.

You can generate fixtures by running:

bin/rake frontend:fixtures to generate all fixtures
bin/rspec spec/frontend/fixtures/merge_requests.rb to generate specific fixtures (in this case for merge_request.rb)

You can find generated fixtures are in tmp/tests/frontend/fixtures-ee.

Creating new fixtures

For each fixture, you can find the content of the response variable in the output file. For example, a test named "merge_requests/diff_discussion.json" in spec/frontend/fixtures/merge_requests.rb produces an output file tmp/tests/frontend/fixtures-ee/merge_requests/diff_discussion.json. The response variable gets automatically set if the test is marked as type: :request or type: :controller.

When creating a new fixture, it often makes sense to take a look at the corresponding tests for the endpoint in (ee/)spec/controllers/ or (ee/)spec/requests/.

GraphQL query fixtures

You can create a fixture that represents the result of a GraphQL query using the get_graphql_query_as_string helper method. For example:

# spec/frontend/fixtures/releases.rb

describe GraphQL::Query, type: :request do
  include GraphqlHelpers

  all_releases_query_path = 'releases/graphql/queries/all_releases.query.graphql'
  fragment_paths = ['releases/graphql/fragments/release.fragment.graphql']

  before(:all) do
    clean_frontend_fixtures('graphql/releases/')
  end

  it "graphql/#{all_releases_query_path}.json" do
    query = get_graphql_query_as_string(all_releases_query_path, fragment_paths)

    post_graphql(query, current_user: admin, variables: { fullPath: project.full_path })

    expect_graphql_errors_to_be_empty
  end
end

This will create a new fixture located at tmp/tests/frontend/fixtures-ee/graphql/releases/graphql/queries/all_releases.query.graphql.json.

You will need to provide the paths to all fragments used by the query. get_graphql_query_as_string reads all of the provided file paths and returns the result as a single, concatenated string.

You can import the JSON fixture in a Jest test using the getJSONFixture method as described below.

Use fixtures

Jest and Karma test suites import fixtures in different ways:

The Karma test suite are served by jasmine-jquery.
Jest use spec/frontend/__helpers__/fixtures.js.

The following are examples of tests that work for both Karma and Jest:

it('makes a request', () => {
  const responseBody = getJSONFixture('some/fixture.json'); // loads spec/frontend/fixtures/some/fixture.json
  axiosMock.onGet(endpoint).reply(200, responseBody);

  myButton.click();

  // ...
});

it('uses some HTML element', () => {
  loadFixtures('some/page.html'); // loads spec/frontend/fixtures/some/page.html and adds it to the DOM

  const element = document.getElementById('#my-id');

  // ...
});

Data-driven tests

Similar to RSpec's parameterized tests, Jest supports data-driven tests for:

Individual tests using test.each (aliased to it.each).
Groups of tests using describe.each.

These can be useful for reducing repetition within tests. Each option can take an array of data values or a tagged template literal.

For example:

// function to test
const icon = status => status ? 'pipeline-passed' : 'pipeline-failed'
const message = status => status ? 'pipeline-passed' : 'pipeline-failed'

// test with array block
it.each([
    [false, 'pipeline-failed'],
    [true, 'pipeline-passed']
])('icon with %s will return %s',
 (status, icon) => {
    expect(renderPipeline(status)).toEqual(icon)
 }
);

NOTE: Only use template literal block if pretty print is not needed for spec output. For example, empty strings, nested objects etc.

For example, when testing the difference between an empty search string and a non-empty search string, the use of the array block syntax with the pretty print option would be preferred. That way the differences between an empty string e.g. '' and a non-empty string e.g. 'search string' would be visible in the spec output. Whereas with a template literal block, the empty string would be shown as a space, which could lead to a confusing developer experience

// bad
it.each`
    searchTerm | expected
    ${''} | ${{ issue: { users: { nodes: [] } } }}
    ${'search term'} | ${{ issue: { other: { nested: [] } } }}
`('when search term is $searchTerm, it returns $expected', ({ searchTerm, expected }) => {
  expect(search(searchTerm)).toEqual(expected)
});

// good
it.each([
    ['', { issue: { users: { nodes: [] } } }],
    ['search term', { issue: { other: { nested: [] } } }],
])('when search term is %p, expect to return %p',
 (searchTerm, expected) => {
    expect(search(searchTerm)).toEqual(expected)
 }
);

// test suite with tagged template literal block
describe.each`
    status   | icon                 | message
    ${false} | ${'pipeline-failed'} | ${'Pipeline failed - boo-urns'}
    ${true}  | ${'pipeline-passed'} | ${'Pipeline succeeded - win!'}
`('pipeline component', ({ status, icon, message }) => {
    it(`returns icon ${icon} with status ${status}`, () => {
        expect(icon(status)).toEqual(message)
    })

    it(`returns message ${message} with status ${status}`, () => {
        expect(message(status)).toEqual(message)
    })
});

Gotchas

RSpec errors due to JavaScript

By default RSpec unit tests don't run JavaScript in the headless browser and rely on inspecting the HTML generated by rails.

If an integration test depends on JavaScript to run correctly, you need to make sure the spec is configured to enable JavaScript when the tests are run. If you don't do this, the spec runner displays vague error messages.

To enable a JavaScript driver in an rspec test, add :js to the individual spec or the context block containing multiple specs that need JavaScript enabled:

# For one spec
it 'presents information about abuse report', :js do
  # assertions...
end

describe "Admin::AbuseReports", :js do
  it 'presents information about abuse report' do
    # assertions...
  end
  it 'shows buttons for adding to abuse report' do
    # assertions...
  end
end

Jest test timeout due to asynchronous imports

If a module asynchronously imports some other modules at runtime, these modules must be transpiled by the Jest loaders at runtime. It's possible that this can cause Jest to timeout.

If you run into this issue, consider eager importing the module so that Jest compiles and caches it at compile-time, fixing the runtime timeout.

Consider the following example:

// the_subject.js

export default {
  components: {
    // Async import Thing because it is large and isn't always needed.
    Thing: () => import(/* webpackChunkName: 'thing' */ './path/to/thing.vue'),
  }
};

Jest doesn't automatically transpile the thing.vue module, and depending on its size, could cause Jest to time out. We can force Jest to transpile and cache this module by eagerly importing it like so:

// the_subject_spec.js

import Subject from '~/feature/the_subject.vue';

// Force Jest to transpile and cache
// eslint-disable-next-line import/order, no-unused-vars
import _Thing from '~/feature/path/to/thing.vue';

NOTE: Do not disregard test timeouts. This could be a sign that there's actually a production problem. Use this opportunity to analyze the production webpack bundles and chunks and confirm that there is not a production issue with the asynchronous imports.

Overview of Frontend Testing Levels

Main information on frontend testing levels can be found in the Testing Levels page.

Tests relevant for frontend development can be found at the following places:

spec/javascripts/, for Karma tests
spec/frontend/, for Jest tests
spec/features/, for RSpec tests

RSpec runs complete feature tests, while the Jest and Karma directories contain frontend unit tests, frontend component tests, and frontend integration tests.

All tests in spec/javascripts/ are intended to be migrated to spec/frontend/ (see also #52483).

Before May 2018, features/ also contained feature tests run by Spinach. These tests were removed from the codebase in May 2018 (#23036).

Test helpers

Test helpers can be found in spec/frontend/__helpers__. If you introduce new helpers, place them in that directory.

Vuex Helper: `testAction`

We have a helper available to make testing actions easier, as per official documentation:

testAction(
  actions.actionName, // action
  { }, // params to be passed to action
  state, // state
  [
    { type: types.MUTATION},
    { type: types.MUTATION_1, payload: {}},
  ], // mutations committed
  [
    { type: 'actionName', payload: {}},
    { type: 'actionName1', payload: {}},
  ] // actions dispatched
  done,
);

Check an example in spec/frontend/ide/stores/actions_spec.js.

Wait until Axios requests finish

The Axios Utils mock module located in spec/frontend/mocks/ce/lib/utils/axios_utils.js contains two helper methods for Jest tests that spawn HTTP requests. These are very useful if you don't have a handle to the request's Promise, for example when a Vue component does a request as part of its life cycle.

waitFor(url, callback): Runs callback after a request to url finishes (either successfully or unsuccessfully).
waitForAll(callback): Runs callback once all pending requests have finished. If no requests are pending, runs callback on the next tick.

Both functions run callback on the next tick after the requests finish (using setImmediate()), to allow any .then() or .catch() handlers to run.

`shallowMountExtended` and `mountExtended`

The shallowMountExtended and mountExtended utilities provide you with the ability to perform any of the available DOM Testing Library queries by prefixing them with find or findAll.

import { shallowMountExtended } from 'helpers/vue_test_utils_helper';

describe('FooComponent', () => {
  const wrapper = shallowMountExtended({
    template: `
      <div data-testid="gitlab-frontend-stack">
        <p>GitLab frontend stack</p>
        <div role="tablist">
          <button role="tab" aria-selected="true">Vue.js</button>
          <button role="tab" aria-selected="false">GraphQL</button>
          <button role="tab" aria-selected="false">SCSS</button>
        </div>
      </div>
    `,
  });

  it('finds elements with `findByTestId`', () => {
    expect(wrapper.findByTestId('gitlab-frontend-stack').exists()).toBe(true);
  });

  it('finds elements with `findByText`', () => {
    expect(wrapper.findByText('GitLab frontend stack').exists()).toBe(true);
    expect(wrapper.findByText('TypeScript').exists()).toBe(false);
  });

  it('finds elements with `findAllByRole`', () => {
    expect(wrapper.findAllByRole('tab').length).toBe(3);
  });
});

Check an example in spec/frontend/alert_management/components/alert_details_spec.js.

Testing with older browsers

Some regressions only affect a specific browser version. We can install and test in particular browsers with either Firefox or BrowserStack using the following steps:

BrowserStack

BrowserStack allows you to test more than 1200 mobile devices and browsers. You can use it directly through the live app or you can install the chrome extension for easy access. Sign in to BrowserStack with the credentials saved in the Engineering vault of the GitLab shared 1Password account.

Firefox

macOS

You can download any older version of Firefox from the releases FTP server, https://ftp.mozilla.org/pub/firefox/releases/:

From the website, select a version, in this case 50.0.1.
Go to the mac folder.
Select your preferred language. The DMG package is inside. Download it.
Drag and drop the application to any other folder but the Applications folder.
Rename the application to something like Firefox_Old.
Move the application to the Applications folder.
Open up a terminal and run /Applications/Firefox_Old.app/Contents/MacOS/firefox-bin -profilemanager to create a new profile specific to that Firefox version.
Once the profile has been created, quit the app, and run it again like normal. You now have a working older Firefox version.

Snapshots

By now you've probably heard of Jest snapshot tests and why they are useful for various reasons. To use them within GitLab, there are a few guidelines that should be highlighted:

Treat snapshots as code
Don't think of a snapshot file as a Blackbox
Care for the output of the snapshot, otherwise, it's not providing any real value. This will usually involve reading the generated snapshot file as you would read any other piece of code

Think of a snapshot test as a simple way to store a raw String representation of what you've put into the item being tested. This can be used to evaluate changes in a component, a store, a complex piece of generated output, etc. You can see more in the list below for some recommended Do's and Don'ts. While snapshot tests can be a very powerful tool. They are meant to supplement, not to replace unit tests.

Jest provides a great set of docs on best practices that we should keep in mind when creating snapshots.

How does a snapshot work?

A snapshot is purely a stringified version of what you ask to be tested on the lefthand side of the function call. This means any kind of changes you make to the formatting of the string has an impact on the outcome. This process is done by leveraging serializers for an automatic transform step. For Vue this is already taken care of by leveraging the vue-jest package, which offers the proper serializer.

Should the outcome of your spec be different from what is in the generated snapshot file, you'll be notified about it by a failing test in your test suite.

Find all the details in Jests official documentation https://jestjs.io/docs/snapshot-testing

How to take a snapshot

it('makes the name look pretty', () => {
  expect(prettifyName('Homer Simpson')).toMatchSnapshot()
})

When this test runs the first time a fresh .snap file will be created. It will look something like this:

// Jest Snapshot v1, https://goo.gl/fbAQLP

exports[`makes the name look pretty`] = `
Sir Homer Simpson the Third
`

Now, every time you call this test, the new snapshot will be evaluated against the previously created version. This should highlight the fact that it's important to understand the content of your snapshot file and treat it with care. Snapshots will lose their value if the output of the snapshot is too big or complex to read, this means keeping snapshots isolated to human-readable items that can be either evaluated in a merge request review or are guaranteed to never change. The same can be done for wrappers or elements

it('renders the component correctly', () => {
  expect(wrapper).toMatchSnapshot()
  expect(wrapper.element).toMatchSnapshot();
})

The above test will create two snapshots, what's important is to decide which of the snapshots provide more value for the codebase safety i.e. if one of these snapshots changes, does that highlight a possible un-wanted break in the codebase? This can help catch unexpected changes if something in an underlying dependency changes without our knowledge.

Pros and Cons

Pros

Speed up the creation of unit tests
Easy to maintain
Provides a good safety net to protect against accidental breakage of important HTML structures

Cons

Is not a catch-all solution that replaces the work of integration or unit tests
No meaningful assertions or expectations within snapshots
When carelessly used with GitLab UI it can create fragility in tests when the underlying library changes the HTML of a component we are testing

A good guideline to follow: the more complex the component you may want to steer away from just snapshot testing. But that's not to say you can't still snapshot test and test your component as normal.

When to use

Use snapshots when

to capture a components rendered output
to fully or partially match templates
to match readable data structures
to verify correctly composed native HTML elements
as a safety net for critical structures so others don't break it by accident
Template heavy component
Not a lot of logic in the component
Composed of native HTML elements

When not to use

Don't use snapshots when

To capture large data structures just to have something
To just have some kind of test written
To capture highly volatile UI elements without stubbing them (Think of GitLab UI version updates)

Return to Testing documentation

Frontend testing standards and style guidelines

Vue.js testing

Jest

Karma test suite

Differences to Karma

Limitations of jsdom

Debugging Jest tests

Timeout error

Test-specific stylesheets

What and how to test

Don't test the library

Don't test your mock

Follow the user

Common practices

How to query DOM elements

Querying for child components

Naming unit tests

Testing promises

Manipulating Time

setTimeout() / setInterval() in application

Waiting in tests

Promises and Ajax calls

Vue rendering

Events

Ensuring that tests are isolated

Jest best practices

Prefer toBe over toEqual when comparing primitive values

Prefer more befitting matchers

Avoid using toBeTruthy or toBeFalsy

Tricky toBeDefined matcher

Avoid using setImmediate

Avoid non-deterministic specs

Faking Date for determinism

Faking Math.random for determinism

Factories

Mocking Strategies with Jest

Stubbing and Mocking

Manual module mocks

Where should I put manual mocks?

Manual mock examples

Keep mocks light

Additional mocking techniques

Running Frontend Tests

Live testing and focused testing -- Jest

Live testing and focused testing -- Karma

Frontend test fixtures

Generate fixtures

Creating new fixtures

GraphQL query fixtures

Use fixtures

Data-driven tests

Gotchas

RSpec errors due to JavaScript

Jest test timeout due to asynchronous imports

Overview of Frontend Testing Levels

Test helpers

Vuex Helper: testAction

Wait until Axios requests finish

shallowMountExtended and mountExtended

Testing with older browsers

BrowserStack

Firefox

macOS

Snapshots

How does a snapshot work?

How to take a snapshot

Pros and Cons

When to use

When not to use

`setTimeout()` / `setInterval()` in application

Prefer `toBe` over `toEqual` when comparing primitive values

Avoid using `toBeTruthy` or `toBeFalsy`

Tricky `toBeDefined` matcher

Avoid using `setImmediate`

Faking `Date` for determinism

Faking `Math.random` for determinism

Vuex Helper: `testAction`

`shallowMountExtended` and `mountExtended`